Aircraft with a double fin and rudder assembly, generally called simply a "rudder assembly" herein, arranged on the fuselage are known, particularly as fighter aircraft. In such an application, the double rudder assembly offers advantages over a more common rudder assembly with only a single stabilizer fin, with regard to aerodynamics and weight. For example, the double rudder assembly reduces roll coupling when the rudder is deflected and provides better or more effective rudder action under flow separation conditions (for example, in a tailspin). A double rudder assembly also has a reduced structural weight, for example by reducing bending moments on the rudders, since the span width of each of the vertical fins in a double rudder assembly is smaller than that of a fin in a single rudder assembly.
Another type of double rudder assembly is known in which rudders are arranged as end disks on the outer ends of the tailplane and elevator assembly that is attached to the tail section of the fuselage. This type of stabilizer assembly can also be constructed as a triple rudder assembly, when an additional center rudder assembly is mounted on the tail section (for example: Lockheed "Constellation"). This type of double or multiple rudder assembly in which rudders extend from the outboard ends of the tailplane and elevator assembly is clearly distinguished from double rudder assemblies that extend solely from the fuselage of the aircraft.
With such a double rudder assembly extending directly from the fuselage, a sufficient distance must be maintained between the two stabilizer fins to avoid aerodynamic interference between the two fins. Generally, this means that the tail section of the fuselage must be wide enough to provide sufficient space to mount the two vertical fins the necessary distance apart. Such a wide tail section is provided, for example, when two tail section jet engines are arranged side-by-side, or when the rudder assemblies extend from the inner tailplane and elevator structure, such as is often the case in present day fighter aircraft.
For various reasons, double rudder assemblies are hardly ever used on aircraft other than fighter aircraft. The tail sections of present day commercial or private aircraft are generally constructed as slender spindle-shaped bodies. This construction makes it difficult to provide double rudder assemblies that extend from the tail section of the fuselage because the fuselage width of the aircraft is generally not wide enough to provide a sufficient spacing between the two rudders. If a double rudder assembly is used, then the rudder assembly is generally a so-called V-rudder assembly and/or the aircraft fuselage tail section is not cone shaped, but rather, ends as a broad flat edge. This construction is not typical of modern commercial aircraft because of increased air flow resistance at the tail section.
An example of an aircraft with a broad flat tail section and V-rudder assembly is described in the magazine FLIGHT 08 of May 14, 1996, whereby the aircraft described is a special military aircraft. V-rudder assemblies have disadvantages that make them difficult to use on commercial aircraft. For example, they give rise to axial coupling in the steering control of the aircraft, which, in commercial transport aircraft causes difficulties above all with the development of effective autopilot systems and with the regulatory approval or certification thereof.
The possibility of extending double rudder assemblies from a tailplane and elevator assembly mounted on top of the fuselage tail section is hardly feasible in commercial aircraft because the tailplanes are tiltable for trimming and thus have a moveable torsion box. The situation is different with fighter aircraft, in which the tailplane and elevator assembly halves are mounted on pivot pins outside the double rudder assemblies. Considerations of weight and aerodynamics, however, make such a solution impractical for commercial aircraft.
The typical tail assembly or empennage of a present day commercial aircraft, unless it is a so-called T-tail assembly which will be discussed below, comprises a vertical rudder assembly fin and an elevator assembly fin or tailplane that are coupled to the tail section cone at approximately right angles to each other, as can be seen in FIG. 1. In order to comply with the area rule that is strived for in the transonic speed range, the cross-section of the tail section cone is constricted where the rudder and elevator assemblies intersect with the tail section, for example in a so-called wasp waist or in the shape of a Coca Cola.RTM. bottle, as shown in FIG. 2. The elevator assembly fin or tailplane should be constructed so as to be trimmable, i.e. pivotable, within a certain angular range. In practical applications, the torsion box of the elevator assembly fin or tailplane penetrates through the tail section of the fuselage and the tailplane is pivotably mounted and supported relative to the tail section by means of bearing mounts and adjusting or positioning jacks. Accordingly, an opening in the tail section must be large enough to accommodate the tailplane torsion box as well as space for the motion of the torsion box in the trimming range of the elevator assembly tailplanes, as shown in FIG. 3. Furthermore, cover plates must be provided on the elevator and tailplane assembly to cover the torsion box opening to prevent airflow into the tail section at any position of the tailplane. Since the opening to be covered is rather large, this is generally achieved only when the form of the ribs in the area around the opening in the tail section is flattened to a certain extent. This, together with the constriction of the tail section results in a spherically curved or irregular surface form that adds to the complexity and cost of manufacturing the aircraft.
The fact that a significant portion of space in the tail section is unavailable for productive use, e.g. as cabin space, is a major disadvantage of the conventional tail section shown in FIGS. 1 to 3. This fact has particularly negative consequences for large high capacity commercial aircraft, and particularly for an aircraft that has two passenger decks, such as is shown in FIGS. 1 and 2. If the tailplane and elevator assembly could be mounted external to the tail section in a different arrangement, then it would be possible to gain additional usable fuselage length in the tail section by as much as a fuselage length AL, shown in FIG. 1. In addition, it would then be possible to construct the tail section of the aircraft with a fuller or wider contour, as indicated by the dotted lines in FIG. 1. This fuller contour also provides additional usable space or fuselage volume, and allows increasing the height of the ribs for introducing tail assembly forces. Furthermore, the contour indicated by the dotted lines in FIGS. 1 and 2 provides a more regular body surface that is simpler and, thus, less costly to manufacture.
A T-tail assembly, that is, an assembly in which the tailplane and elevator assembly is mounted on the upper end of the vertical fin and rudder assembly, provides a way of arranging the tailplane and elevator assembly external to the tail section. One consequence of the T-tail assembly is that the forces on the tailplane and elevator assembly must be transmitted and introduced through the vertical fin into the tail section cone. The connecting ribs for the vertical fins must be dimensioned accordingly, that is, they must be larger in order to carry the additional forces from the tailplane and elevator assembly. Heavier dimensioned ribs increase the weight and also interfere with the amount of cabin space that can be used in the tail section because of their larger dimensions. Consequently, the additional tail section space gained from eliminating the torsion box of the tailplane and elevator assembly from the tail section is not fully available for use as productive cabin space.